*{
    box-sizing: border-box;
}

#heart{
    display: inline-block;
    margin: 100px;
    position: relative;
    animation: .5s heart infinite alternate-reverse;
}
/* 注意英文单词的拼写。刚才没显示出来是因为将alternate拼为alternative */

@keyframes heart{
    0%{
        transform: scale(1);
    }
    100%{
        transform: scale(1.2);
    }
}

#heart>.left{
    background : red;
    width : 50px;
    height: 50px;
    position: absolute;
    transform: rotate(45deg) translateX(31px);
    bottom : 50px;
    left: -50px;
    border-radius: 50% 0 0 50%;
}


#heart>.right{
    background : red;
    width : 50px;
    height: 50px;
    border-radius: 50%;
    position: absolute;
    transform: rotate(45deg) translateY(31px);
    bottom : 50px;
    right: -50px;
    border-radius: 50% 50% 0 0;
}

#heart>.bottom{
    background : red;
    width : 50px;
    height: 50px;
    transform: rotate(45deg);
}